Several approaches to remotely collecting data from meter reading monitoring stations, such as those for gas and electric power meters, have been proposed.
In one system described in U.S. Pat. No. 4,614,945 to Brunius et al., multiple, battery-powered monitoring stations, such as gas meter monitors, are located throughout an area and each of the monitoring stations includes a transponder (referred to by Brunius as an Encoder/Receiver/Transmitter unit ("ERT")) for transmitting radiowave signals corresponding to data collected by the monitoring station. A mobile data collection unit collects data by traveling through the area, activating the ERTs to transmit their radiowave signals and receiving and decoding the radiowave signals to identify the data. To activate the ERTs, the mobile unit includes a transmitter that emits a "wake-up" or "interrogation" signal. All ERTs within range of the mobile unit, upon receiving the interrogation signal, respond by transmitting their accumulated data by transmitting their identification codes and accumulated data a plurality of times by means of serially spaced bursts.
At times, the mobile unit may be within range of several ERTs simultaneously and, because the mobile unit does not uniquely poll the individual ERTs, it may energize all ERTs within range of the wake-up signal simultaneously. Because more than one ERT may begin transmission at the same time, their signals may "collide" at the mobile unit. That is, radiowave signals from several of the transponders may arrive at the mobile unit together, such that the receiver within the mobile unit receives a combination of the various signals from the transponders. In such a case, the signals may be difficult to detect or may result in an incorrect signal being detected by the receiver.
Brunius treats the problem of signal collision by having the time interval between successive transmission bursts be determined as a function of the identification code of the transponder unit such that, with each transponder being assigned an identification code differing from other transponders in the area, the temporal spacing between bursts will differ between transponders. Nevertheless, the initial bursts may occur at the same time.
Brunius also varies transponder frequency to address the problem of collisions. Each transponder, upon receiving the wake-up signal, begins transmission at a preset transmission frequency and successive transmissions by the transponder are shifted to different frequencies. The frequency shift of successive transmissions is dependent upon the time interval, and thus the unit identification number.
Brunius does not teach a method of varying the frequency of the initial bursts. Instead, Brunius relies upon "tuning variances" to give differing initial frequencies of transmission. Because the Brunius system requires transmission and reception of wake up signal, it requires the mobile unit to include both a receiver and a transmitter and also requires the monitoring station to include a transponder to both transmit and receive data. Thus, though data transfer is unidirectional (from the monitor to the receiver) both the monitor and the receiver require transponders.
Because the Brunius units are battery powered, the units remain inactive in the absence of the "wake-up" signal to conserve power. Even though this intermittent activity may conserve some power, the batteries within the units still must be replaced eventually. Often, such replacement of the batteries requires removal of the monitoring meter. Governmental regulations require meters to be recalibrated whenever they are removed, so battery replacement typically involves returning the meter to a repair facility for recalibration.
An additional problem that must be addressed in remote data collection systems is confinement of transmission with specified frequency limits. The amount of allowable frequency variation is limited by various factors. For example, the available frequency band is prescribed by governmental regulation and depends upon the characteristics of the transmitters and receivers.
It is therefore necessary to control the output frequency of the transmitters within the system such that transmissions are limited to a predetermined frequency range. While the frequency range may be defined somewhat by the design of the system, tight control of the output frequency is often difficult without complex systems, such as those using precision components or feedback configurations.
Moreover, because the monitors may operate over a wide range of operating environments, including wide temperature variations, component values may vary. Consequently, the output frequencies may drift outside of the design limits as the operating temperature changes.